Power transducer

ABSTRACT

The performance of a power transducer is improved while efficiently using a power semiconductor also by managing the permissible duty factor of the power semiconductor in the regenerative braking circuit provided in the power transducer. The user is allowed to set, through an operation panel provided on the power transducer, the resistance value of the regenerative braking resistor for thermally consuming the rotational energy generated during motor deceleration. The power transducer performs the steps of: calculating the current which flows in the regenerative braking circuit from the resistance value setting; obtaining the generation loss of the power semiconductor in the regenerative braking circuit with the calculated current value; and determining the permissible duty factor of the power semiconductor from the obtained generation loss.

CLAIM OF PRIORITY

The present application claims priority from Japanese application serialno. JP2008-058918, field on Mar. 10, 2008, the content of which ishereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power transducer.

2. Description of the Related Art

Inverters, a form of electrical power transducer, have been widelyadopted in industries and also used in household electric appliances asrotational velocity control devices for AC motors. When the rotor of anAC motor is decelerated, the rotational energy of the AC motor duringthe deceleration is accumulated as electrostatic energy in a smoothingcapacitor located in the DC intermediate circuit of its transducer.However, voltages may increase at both ends of the smoothing capacitordue to its small capacitance. This puts into operation the overvoltageprotection circuit provided in the DC intermediate circuit of thetransducer, bringing the transducer to a halt.

For this reason, a power transducer is provided with a regenerativebraking resistor in its DC intermediate circuit so that the regenerativebraking resistor consumes the deceleration rotational energy of the ACmotor as thermal energy.

The rated currents of the regenerative braking resistor and of powersemiconductors in the regenerative braking circuit have upper limits. Ifused beyond the limits, the regenerative braking resistor and the powersemiconductors receive extremely high voltages: they may be thermallydamaged due to electrical heat generation. Thus, a maximum duty factor,a quantitative attribute, is predefined for the regenerative brakingresistor (the maximum duty factor is hereinafter referred to as % ED).

Therefore, the power transducer manages the cumulative operating time ofthe regenerative braking circuit provided in its DC intermediatecircuit. When the cumulative operating time percentage exceeds the % EDof the regenerative braking resistor, the power transducer disconnects aswitching element in the regenerative braking circuit provided in the DCintermediate circuit to cut off the current flowing in the regenerativebraking resistor, thus protecting the regenerative braking resistor fromthermal destruction due to electrical heat generation.

However, no strict definition has been given to the permissible dutyfactor % ED_(pm) of the power semiconductor (as a switching element) ofthe regenerative braking circuit provided in the DC intermediate circuitin the power transducer. Only the % ED_(R) is determined based mainly onthe correlation with the regenerative braking resistor.

A power semiconductor used for the power transducer, such as IGBT,generates heat because of electrical loss generated during electricpower conversion. Therefore, operating the power transducer beyond thesemiconductor's operating limit temperature may result in thermaldestruction and deactivation. Therefore, the power transducer includes acooling fin and a cooling fan for cooling the power semiconductor. Heatis conducted from the power semiconductor to the cooling fin, and thecooling fan sends air to the cooling fin for thermal exchange to allowheat radiation.

Paragraph 0007 of Japanese Patent No. 3648932 describes that thevoltage-type power transducer comprises: a series circuit connected atboth ends of the smoothing capacitor, the series circuit consisting of abraking resistor and a transistor switch; a voltage detecting circuitwhich closes the switch when the voltage at both ends of the smoothingcapacitor exceeds a predetermined value; a totalizing circuit whichtotalizes time intervals during which the switch is closed within apredetermined time period (T) based on a start command from outside, andoutputs the total time (Σt); a usage rate calculation circuit whichcalculates the usage rate (X %: X=(Σt/T)*100) of the braking resistorfrom the predetermined time period (T) and total time (Σt); and adisplay circuit which displays the usage rate (X %) when thepredetermined time period (T) has elapsed.

Further, JP-A-5-168287 and JP-A-10-229607 describe that overloadprotection and overheat protection are performed for the regenerativebraking resistor.

SUMMARY OF THE INVENTION

According to the description of the above-mentioned conventionaltechnique, although overload protection and overheat protection areperformed by managing the permissible duty factor % ED of theregenerative braking resistor, no consideration is given to % EDmanagement for the power semiconductor in the regenerative brakingcircuit.

Therefore, the power semiconductor generates heat because of losselectrically generated during electric power conversion and, operatingthe power transducer beyond the semiconductor's operating limittemperature may result in thermal destruction and deactivation. Noconsideration has been given to this problem.

An object of the present invention is to improve the performance of thepower transducer while efficiently using the power semiconductor also bymanaging the permissible duty factor % ED of the power semiconductor inthe regenerative braking circuit provided in the power transducer.

In order to attain the above object, the present invention specificallyprovides a power transducer which outputs AC power having a variablevoltage and a variable frequency, the power transducer comprising: aconverter which rectifies the AC voltage of the AC power supply toconvert it to a DC voltage; a DC intermediate circuit having a smoothingcapacitor for smoothing the DC voltage of the converter; a detectingcircuit which detects the voltage of the DC intermediate circuit; aregenerative braking circuit provided in the DC intermediate circuit; atimer which accumulates the operating time of the regenerative brakingcircuit to obtain its cumulative operating time; and an inverter whichconverts the DC voltage of the converter to an AC voltage. It is madepossible to individually set the permission duty factor % ED andresistance value R of the regenerative braking resistor connected to theregenerative braking circuit included in the DC intermediate circuit.

Further, more desirable specific modes according to the above-mentionedaspect are shown below.

(1) Includes an operation panel for setting the % ED_(R) and resistancevalue R of the regenerative braking resistor, and a display unit isprovided on the operation panel.

(2) Includes a nonvolatile memory for storing correlation data of the %ED_(pm) of the power semiconductor in the regenerative braking circuitcorresponding to the resistance value R of the regenerative brakingresistor obtained in advance through calculation.

(3) The % ED_(pm) of the power semiconductor in the regenerative brakingcircuit corresponding to the resistance value setting R of theregenerative braking resistor can be read from the nonvolatile memory.

(4) Includes a drive circuit for activating the regenerative brakingcircuit when the voltage detected by the detecting circuit reaches orexceeds a specified voltage.

(5) Includes a timer which accumulates the operating time (t) of theregenerative braking circuit to obtain its cumulative operating time(Σt).

(6) When the cumulative operating time percentage (Σt/T*100) obtained bythe timer reaches the % ED_(R) of the regenerative braking resistor orthe % ED_(pm) of the power semiconductor in the regenerative brakingcircuit, whichever smaller, only the regenerative braking circuit isdisconnected without breaking the output of the power transducer.

(7) The cumulative operating time obtained by the timer is automaticallycleared (to zero) after a specified time T has elapsed.

(8) When only the regenerative braking circuit is disconnected, thefailure is displayed on the display unit of the operation panel.

In accordance with the present invention, it becomes possible to managethe permissible duty factor % ED of the power semiconductor in theregenerative braking circuit provided in the power transducer, thusimproving the performance of the power transducer while efficientlyusing the power semiconductor in the regenerative braking circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a main circuit configuration of a powertransducer.

FIG. 2 is a diagram showing an example layout of main circuit componentsof the power transducer.

FIG. 3 is a diagram showing the route of a current flowing in aregenerative braking circuit.

FIG. 4 is a diagram showing example correlation data between theresistance value setting and the permissible duty factor % ED of theIGBT in the regenerative braking circuit, the data being stored in anonvolatile memory.

FIG. 5 is a diagram showing example % ED_(R) and resistance valuesettings of a regenerative braking resistor.

FIG. 6 is a diagram showing a correction between the resistance valuesetting R and the permissible duty factor % ED of the IGBT in theregenerative braking circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Specific embodiments of the present invention will be explained belowwith reference to FIGS. 1 to 6. However, the present invention is notlimited to the accompanying drawings.

First Embodiment

FIG. 1 is a schematic view showing the circuit configuration of a powertransducer according to the present embodiment. The power transducer 10includes: a converter 1 which converts AC power to DC power; a smoothingcapacitor 2 located in the DC intermediate circuit of the powertransducer 10; an inverter 3 which inverts DC power to AC power at anarbitrary frequency; and an AC motor 4.

The power transducer 10 further includes: a cooling fan 6 which coolsthe converter and a power module in the inverter; and a digitaloperation panel 7 used to set and change various control data of thepower transducer, and display and monitor abnormal states.

The power transducer 10 further includes a control circuit 5 whichincludes a microcomputer (control and calculation unit) to control aswitching element of the inverter and at the same time performs totalcontrol of the power transducer, thus allowing required controlprocessing based on various control data input through the digitaloperation panel 7. The control circuit 5 controls the switching elementof the inverter 3 based on various control data input through thedigital operation panel 7, and performs control processing required forthe entire power transducer.

Although the internal configuration is omitted, the power transduceralso includes a microcomputer (control and calculation unit) whichperforms calculation based on the information from the data stored inthe storage unit for storing various control data. The power transducer10 further includes a driver circuit 8 which drives the switchingelement of the inverter. The driver circuit 8 drives the switchingelement of the inverter 3 and the switching element in the regenerativebraking circuit 9 based on a command from the control circuit 5. Whenthe detection value of a voltage V_(PN) detecting circuit of the DCintermediate circuit reaches or exceeds a specified voltage, the drivercircuit 8 drives the switching element in the regenerative brakingcircuit 9. Further, the microcomputer included in the control circuit 5accumulates the operating time of the regenerative braking circuit bymeans of the timer to obtain the cumulative operating time.

When the cumulative operating time percentage obtained by the timerexceeds a specified % ED, the microcomputer deactivates only theregenerative braking circuit without breaking the output of the powertransducer, and displays the failure on the digital operation panel 7.Further, the driver circuit 8 includes a switching regulator circuit(DC/DC converter) to generate DC voltages required for the operation ofpower transducer, and supply each DC voltage to each component. Thepower transducer 10 further includes a regenerative braking circuit 9 inthe DC intermediate circuit.

The regenerative braking circuit 9 includes an IGBT which is a typicalswitching element. Needless to say, this element is not limited to anIGBT and may be any element having a function of a switching element. Aregenerative braking resistor BR thermally consumes the rotationalenergy generated during AC motor deceleration.

Since the rated current of the IGBT included in the regenerative brakingcircuit 9 has an upper limit, the minimum permissible resistance valueRmin of the regenerative braking resistor BR connectable to theregenerative braking circuit is predetermined as a product specificationfor each capacity of the power transducer in order to prevent the IGBTfrom being destroyed. The minimum permissible resistance value Rmin isoften listed in the product catalogs and operation manuals of powertransducers.

Various control data of the power transducer can be set and changed fromthe operation panel 7. The operation panel 7 includes a display unitcapable of failure display. When a failure is detected in the powertransducer, it is displayed on the display unit. Although the operationpanel 7 in the present embodiment is not limited to a certain type, theoperator can perform its operation as a digital operation panel whilemonitoring the display unit in consideration of the operability for theuser of the power transducer.

Although the display unit is not necessarily built in the operationpanel 7, it is desirable that the display unit be incorporated in theoperation panel 7 to allow the operator of the operation panel 7 toperform operations while monitoring the display.

Various control data of the power transducer input from the operationpanel 7 are stored in a storage unit (not shown). Detailed explanationof an power transducer which is a power transducer will be omitted sinceit is well-known technology.

FIG. 2 is an example layout of main circuit components of the powertransducer. The power semiconductor 11 is a combined module whichincludes the converter 1, the inverter 3, and the regenerative brakingcircuit 9. The power transducer 10 further includes a cooling fin 12 anda cooling fan 6 to cool the power semiconductor 11 configured as acombined module. Heat is conducted from the power semiconductor 11 (heatgenerating apparatus) to the cooling fin 12, and the cooling fan 6 sendsair to the cooling fin 12 for thermal exchange to allow heat radiation.

The power transducer 10 further includes a main circuit substrate 14 anda resin mold case 13. The power semiconductor 11 configured as acombined module further includes a temperature detector to detect thetemperature in the power module. This temperature detector is composedof a thermistor whose resistance value changes with the temperature.

FIG. 3 is a diagram showing the route of a current flowing in theregenerative braking circuit 9 during operation. When the AC motor 4driven by the power transducer 10 is decelerated, the rotational energyis fed back to the smoothing capacitor 2 in the power transducer whichis a power supply because of the inertia of the AC motor which is a bodyof revolution. In this case, the smoothing capacitor 2 accumulates therotational energy as electrostatic energy. The power transducer isprovided with the overvoltage protection function. With large rotationalenergy, the voltage V_(PN) at both ends of the smoothing capacitorincreases. When the detection value of the voltage V_(PN) reaches orexceeds a specified voltage V_(pnd0), this function disconnects theoutput of the power transducer to protect the smoothing capacitor 2 fromovervoltage.

Therefore, in order to avoid the operation of the overvoltage protectionfunction, the power transducer is provided with the regenerative brakingcircuit 9 so that the rotational energy generated during AC motordeceleration is thermally consumed by the regenerative braking resistorBR. Dotted lines of FIG. 3 show the route of the current which flows inthe regenerative braking circuit 9 so that the rotational energygenerated during AC motor deceleration is thermally consumed by theregenerative braking resistor BR.

The operation of the regenerative braking circuit will be explainedbelow. The regenerative braking circuit constantly detects and monitorsthe voltage V_(PN) at both ends of the smoothing capacitor 2 in the DCintermediate circuit and, when the detection value of the voltageV_(PND) reaches or exceeds a specified voltage V_(pnd1), the IGBT(switching element) in the regenerative braking circuit 9 is driven bythe driver circuit 8. Further, when the detection value of the voltageV_(PND) falls below a specified voltage V_(pnd2), the IGBT (switchingelement) in the regenerative braking circuit 9 is disconnected by thedriver circuit 8. In this case, the cumulative operating time of theregenerative braking circuit is obtained by the timer.

Here, detection voltages V_(pnd0), V_(pnd1) and V_(pnd2) of the voltagedetecting circuit provided in the DC intermediate circuit constantlysatisfy a condition V_(pnd0)>V_(pnd1)>V_(pnd2). Although the rotationalenergy generated during AC motor deceleration is thermally consumed bythe regenerative braking resistor BR, the regenerative braking resistorBR cannot thermally consume infinite energy. Since the regenerativebraking resistor consumes the rotational energy of the AC motor byconverting it to thermal energy, operating the power transducer beyondthe semiconductor's operating limit temperature may result in burnout orfire due to abnormal heat generation.

Needless to say, the energy is represented by the product of the powerconsumption and time (V_(pnd) ²/R*t). That is, how long the powerconsumption (V_(pnd) ²/R) can be permitted is determined by thespecification of the regenerative braking resistor. A percentageobtained by dividing the permissible accumulative application time (Σt)by a predetermined time period (T) equals the % ED (% ED=Σt/T*100).

This means that, for example, when the permissible duty factor of theregenerative braking resistor is 5% ED and the time period (T) is 100seconds, the power can be applied for five seconds (5/100*100) asaccumulative application time. In this way, power transducer makers sellas an option regenerative braking resistors having predeterminedresistance and % ED values as specifications.

Specifically, when a maker genuine regenerative braking resistor ispurchased, the resistor cannot be used beyond the specified % ED. Sincethe number of resistance values (% ED specifications) of the makergenuine regenerative braking resistor is limited, the maker offerslimited number of types of regenerative braking resistors.

Accordingly, there are many cases when the user designs the resistanceand % ED values of the regenerative braking resistor suitable formechanical system operation and, if a maker genuine part is notavailable, request a specialized resistor producer to produce theresistor. In this case, the user must design a regenerative brakingresistor BR having a minimum permissible resistance value (Rmin) equalto or larger than that described in the product catalog and operationmanual of the power transducer.

If the resistance value of the designed regenerative braking resistor BRsatisfying the minimum permissible resistance value (Rmin) is not foundin maker genuine parts, it is necessary that the regenerative brakingresistor BRA requested to the specialized resistor producer or anotherbraking resistor BRB stored by the user be connected to the regenerativebraking circuit. In this case, in order to protect the regenerativebraking resistor BR from electrical heat generation, the user must setthe permissible duty factor % ED_(R) of the regenerative brakingresistor through the digital operation panel 7.

This situation, however, causes a problem that the user cannot determinethe permissible duty factor % ED_(pm) of the IGBT included in theregenerative braking circuit with the resistance value R of theregenerative braking resistor BRA or BRB, or whether or not the settingof the permissible duty factor % ED_(R) of the regenerative brakingresistor is thermally acceptable.

Therefore, the user is allowed to set, through the digital operationpanel 7, the permissible duty factor % ED_(R) and resistance value R ofthe new regenerative braking resistor designed by the user. Thisconfiguration makes it possible to easily obtain from the followingformula 1 a current I which flows in the IGBT included in theregenerative braking circuit based on the resistance value R of theregenerative braking resistor set through the digital operation panel 7by the user.

I=V _(PND) /R  (Formula 1)

where V_(PND) is the detection value of the voltage V_(PN) of the DCintermediate circuit, and is a predetermined voltage (V_(PND)=V_(pnd1))for driving the switching element in the regenerative braking circuit 9.The microcomputer (control and calculation unit) included in the controlcircuit 5 performs the steps of: calculating the current I by using theabove-mentioned formula; calculating the generation loss of the IGBTincluded in the regenerative braking circuit 9 based on the calculatedcurrent I; and obtaining the permissible duty factor % ED_(pm) of theIGBT.

Generally, the generation loss P of the IGBT is represented by thefollowing formula 2.

P=Pon+Poff+Psat  (Formula 2)

where Pon denotes the ON loss when the IGBT (power semiconductor)changes from the OFF state to the ON state, Poff denotes the OFF losswhen it changes from ON state to the OFF state, and Psat the steadyloss.

A detailed formula for the generation loss P will be omitted since it isdisclosed in semiconductor maker's application note or the like andtherefore well-known.

The detailed formula for the generation loss of the power semiconductoris prestored in a nonvolatile memory. The microcomputer performs thesteps of: calculating a current (I=V_(PND)/R) which flows in the IGBTincluded in the regenerative braking circuit from the resistance valuesetting R of the regenerative braking resistor; calculating thegeneration loss of the IGBT by using the formula stored in the memory;obtaining the permissible duty factor % ED_(pm) of the IGBT from thecalculated operation loss; and displaying it on the digital operationpanel 7.

The microcomputer compares the permissible duty factor % ED_(R) of theregenerative braking resistor set through the digital operation panel 7by the user with the permissible duty factor % ED_(pm) of the IGBTincluded in the regenerative braking circuit obtained from theresistance value setting R of the regenerative braking resistor. Whenthe cumulative operating time percentage (Σt/T*100) obtained by thetimer reaches the % ED_(R) of the regenerative braking resistor or the %ED_(pm) of the power semiconductor, whichever smaller, the microcomputerautomatically disconnects only the power semiconductor in theregenerative braking circuit without breaking the output of the powertransducer.

In this case, when the resistance value setting R of the regenerativebraking resistor is equal to or less than the minimum permissibleresistance value (data stored in a storage unit not shown) predeterminedfor each capacity of the power transducer, the permissible duty factorof the IGBT is displayed as 0% ED on the digital operation panel 7. Thepermissible duty factor % ED_(pm) of the IGBT is displayed on thedigital operation panel 7 aiming at notifying whether or not theresistance value R of the regenerative braking resistor set by the useris acceptable. The display is not necessarily 0% ED. Neither 0 displaynor 0% display impairs the intention of the present invention.

For example, it is possible either to clearly display an incorrectresistance value setting or display NG-R which means an incorrectresistance value setting. The display contents are not limited as longas its meaning can be recognized by the operator of the powertransducer.

Since displaying results on the digital operation panel 7 in this wayallows the operator to recognize various settings, the operator canclearly judge the situation from the display contents.

In this case, the permissible duty factor % ED_(pm) of the IGBT includedin the regenerative braking circuit 9 obtained from the resistance valuesetting R of the regenerative braking resistor is smaller than the %ED_(R) of the regenerative braking resistor and becomes 0%. This meansthat the regenerative braking circuit does not operate at all. Thisphenomenon occurs because the selected resistance value R of theregenerative braking resistor is equal to or less than the minimumpermissible resistance value Rmin predetermined for each capacity of thepower transducer. This function is aimed at the prevention ofdestruction of the IGBT included in the regenerative braking circuit 9.

FIG. 4 shows another embodiment which performs the steps of: calculatingcorrelation data of the permissible duty factor % ED_(pm) of the IGBT inthe regenerative braking circuit corresponding to the resistance value Rof the regenerative braking resistor; storing the calculation result inthe nonvolatile memory; and reading the data from this memory.

The loss calculation simulation software for the generation loss P ofthe power semiconductor can also be downloaded at the websites of powersemiconductor makers.

The use of this simulation software makes it easier to obtain thecurrent I=V_(PND)/R which flows in the power semiconductor from theresistance value setting R of the regenerative braking resistor,calculate the generation loss P of the power semiconductor under anygiven operating conditions, and obtain in advance the permissible dutyfactor % EDM of the IGBT corresponding to the resistance value setting Rfrom the calculated generation loss.

FIG. 4 shows an example correlation data between the resistance valuesetting R of the regenerative braking resistor and the permissible dutyfactor % ED_(pm) of the IGBT in the regenerative braking circuit, thedata being stored in the nonvolatile memory. Although the data is storedin memory as binary numbers, the data is represented by decimal numbersfor easier understanding. This does not impair the intention of thepresent invention. Further, each numerical value described is merely anexample and is not limited thereto.

The correlation data between the resistance value setting R of theregenerative braking resistor and the permissible duty factor % ED_(pm)of the IGBT in the regenerative braking circuit is prestored in thenonvolatile memory. The permissible duty factor % ED_(pm) of the IGBTcorresponding to the resistance value setting R of the regenerativebraking resistor is read from the nonvolatile memory.

Judging by the load factor of the microcomputer, it is not realisticthat the microcomputer calculates the complicated generation loss of thepower semiconductor. It is more realistic that the microcomputer storesin the nonvolatile memory the correlation data between the resistancevalue setting R of the regenerative braking resistor and the permissibleduty factor % ED_(pm) of the IGBT in the regenerative braking circuit,and reads from the nonvolatile memory the permissible duty factor %ED_(pm) of the IGBT corresponding to the resistance value setting R ofthe regenerative braking resistor.

FIG. 5 shows example % ED_(R) and resistance value R of the regenerativebraking resistor BR set through the digital operation panel 7 by theuser. Although the regenerative braking resistor set by the user has a %ED_(R) (20.0/resistance value R) of 3.6 ohms, The permissible dutyfactor % ED_(pm) of the IGBT in the regenerative braking circuitcorresponding to the resistance value is the value read from thenonvolatile memory of FIG. 4, that is, % ED_(pm)=3.3.

In this case, the microcomputer in the control circuit compares %ED_(R)=20.0 of the regenerative braking resistor with the permissibleduty factor % ED_(pm)=3.3 of the IGBT in the regenerative brakingcircuit read from the nonvolatile memory. When the cumulative operatingtime percentage (Σt/T*100) of the control circuit reaches one of the twovalues (% ED_(R)=20.0 and % ED_(pm)=3.3), whichever smaller, i.e., whenΣt/T*100 reaches % ED_(pm)=3.3, the microcomputer deactivates only theregenerative braking circuit and then displays the failure on thedigital operation panel 7.

FIG. 6 shows example correlation between the resistance value setting Rof the regenerative braking resistor and the permissible duty factor %ED_(pm) of the IGBT in the regenerative braking circuit. The datacurving line value of the correlation diagram is stored in thenonvolatile memory.

The horizontal axis is assigned the resistance value setting R of theregenerative braking resistor, and the vertical axis the permissibleduty factor % ED_(pm) of the IGBT in the regenerative braking circuit.With the minimum permissible resistance value Rmin of the regenerativebraking resistor, the permissible duty factor % ED_(pm) of the IGBT is %EDmin; with a resistance value less than Rmin, the permissible dutyfactor % ED_(pm) of the IGBT is 0%. This means that the IGBT in theregenerative braking circuit cannot operate with a resistance value lessthan the minimum permissible resistance value Rmin.

Further, when the resistance value is R₁₀₀, the permissible duty factor% ED_(pm) of the IGBT is 100%. This means that the IGBT in theregenerative braking circuit can continuously operate when theresistance value R is equal to or larger than R₁₀₀.

In FIG. 6, although the relation between the resistance value setting Rof the regenerative braking resistor and the permissible usage rate %ED_(pm) of the IGBT in the regenerative braking circuit is approximatedto a primary curve, it may be an n-th order curve. This does not impairthe intention of the present invention.

In accordance with the present invention, the % ED_(R) and resistancevalue R of the regenerative braking resistor BR are set through thedigital operation panel 7; the permissible duty factor % ED_(pm) of theIGBT in the regenerative braking circuit corresponding to the resistancevalue setting R is automatically read and displayed on the digitaloperation panel 7; when the cumulative operating time percentage (Σt/T)of the control circuit reaches one of the two values (% ED_(R) and %ED_(pm)), whichever smaller, only the regenerative braking circuit isdeactivated; and the failure is displayed on the digital operation panel7. Therefore, it becomes possible to manage the permissible duty factor% ED_(pm) of the IGBT (power semiconductor) in the regenerative brakingcircuit, thus improving the performance of the power transducer whileefficiently using the power semiconductor in the regenerative brakingcircuit.

1. A power transducer which outputs AC power having a variable voltageand a variable frequency, the power transducer comprising: a converterfor rectifying the AC voltage of an AC power supply to convert it to aDC voltage; a DC intermediate circuit having a smoothing capacitor forsmoothing the DC voltage of the converter; a detecting circuit fordetecting the voltage of the DC intermediate circuit; a regenerativebraking circuit provided in the DC intermediate circuit; a drive circuitfor driving the regenerative braking circuit when the detecting circuitdetects a specified voltage; a timer for accumulating the operating timeof the regenerative braking circuit to obtain the cumulative operatingtime; and an inverter for converting the DC voltage of the converter toan AC voltage, wherein it is possible to individually set a permissibleduty factor % ED and a resistance value R of a regenerative brakingresistor connected to the regenerative braking circuit provided in theDC intermediate circuit.
 2. The power transducer according to claim 1,wherein the power transducer performs the steps of: calculating thecurrent which flows in a power semiconductor, a switching element, inthe regenerative braking circuit from the resistance value setting ofthe regenerative braking resistor; obtaining the generation loss of thepower semiconductor from the calculated current value, and; calculatingthe permissible duty factor of the power semiconductor.
 3. The powertransducer according to claim 1, further comprising: a nonvolatilememory for prestoring correlation data between the generation loss ofthe power semiconductor in the regenerative braking circuit and thepermissible duty factor of the power semiconductor from the resistancevalue setting of the regenerative braking resistor, wherein the powertransducer reads from the nonvolatile memory the permissible duty factordata of the power semiconductor corresponding to the resistance valuesetting of the regenerative braking resistor.
 4. The power transduceraccording to claim 2, wherein the power transducer performs the stepsof: comparing the permissible duty factor of the regenerative brakingresistor with the permissible duty factor of the power semiconductorobtained from the resistance value setting of the regenerative brakingresistor, and; automatically disconnecting only the power semiconductorin the regenerative braking circuit without breaking the output of thepower transducer when the cumulative operating time obtained by thetimer reaches the permissible duty factor of the regenerative brakingresistor or the permissible duty factor of the power semiconductor,whichever smaller.
 5. The power transducer according to claim 2, furthercomprising: a display unit for displaying the permissible duty factor ofthe power semiconductor calculated or read from the nonvolatile memory.6. The power transducer according to claim 1, wherein, when theresistance value setting of the regenerative braking resistor is equalto or less than a predetermined minimum permissible resistance value,the permissible duty factor 0% ED of the power semiconductor isdisplayed on the display unit.
 7. The power transducer according toclaim 1, wherein, when the resistance value setting of the regenerativebraking resistor is equal to or less than the predetermined minimumpermissible resistance value, symbols or characters for indicating aresistance value setting failure are displayed on the display unit. 8.The power transducer given in claim 5, further comprising: an operationpanel provided in the operation panel for allowing the operator toindividually set the permissible duty factor and resistance value of theregenerative braking resistor.